Numerical simulation of a tropical Storm boundary layer

A model has been designed to study the surface boundary layer of a tropical storm. The numerical method consists of solving a two point boundary value problem for two systems of simultaneous non-linear differential equations by finite differences. A Stoke's stream function suitable to represent the flow both in interior and exterior regions of a tropical storm boundary layer has been developed. The advantage of the method is that the, boundary layer of the tropical storm can be studied starting from the outer region to the centre of the storm without neglecting non-linear terms. In addition, there IS no need for assumptions on the vertical profiles for tangential and radial velocities. The method is stable and converges within a few iterations. The flow above the friction layer is represented by a steady axisymmetric vortex in gradient balance. To investigate the effect of turbulence- on boundary layer characteristics, turbulence has been represented by four different variations of the eddy coefficient of viscosity with no slip boundary conditions. Computations have been performed 1aking 40-grid points in the vertical direction. It is observed that, if the eddy coefficient of viscosity is assumed to vary with the superimposed flow above the boundary layer, the solutions compare favourably well with observations. The solution also shows an outflow from the Inner core of the boundary layer which is necessary for creation of an eye of the storm.

Sub-seasonal variations of the tropical storm track in the western north Pacific

With 20-year (1975-94) climatological data, we demonstrate that the tropical storm track over the western North Pacific (0° - 40°N, 100 - 180°E) exhibits prominent sub-seasonal variations on a time scale of about 40 days from May to November. The storm track variability is regulated by the conspicuous Climatological Intra Seasonal Oscillation (CISO) in the strength of the western North Pacific summer monsoon and the associated position of the western Pacific Sub-tropical High. The CISO cycle regulates the number of tropical storm formation during the Pre-Onset and Withdraw Cycles but not during the Onset and Peak Monsoon Cycles (from mid-June to mid-September).

Radial and vertical velocity field in a steady state symmetric tropical storm

ABSTRACT. A method to construct a consistent structure of steady state symmetric tropical storms from a few known values of temperature anomaly in the centre and around it has been developed. The role of kinematic eddy coefficient of viscosity in producing the transverse circulation in a tropical storm has been tested and discussed. The well known features and characteristics of a tropical storm, such as, eyewall, sinking motion, inside the eyewall, low-level radial inflow and high level outflow are well produced in the model. The computation shows that there is an increase of transverse circulation with increase of the magnitude of eddy coefficient. In the boundary layer, the vertical eddy coefficient plays more important role than the radial eddy coefficient; while in the upper layer the latter is much more important than the former. It has also been found that in absence of radial exchange coefficient, there can be no sinking motion in the central region of the storm. The magnitude of radial and vertical wind in the eye region is more sensitive to the variation of radial eddy coefficient. In addition to the eddy coefficients, transverse circulations also depend upon the tangential wind distribution above the boundary layer.

Impact of parameterized convection on a numerically simulated tropical Cyclone structure

ABSTRACT. Condensational heating is a primary source of energy for disturbances like a tropical storm. The resolvable scale condensation and the parameterized convection, in many fine mesh numerical models, are evaluated at intervals greater than the time step, order of a minute, used for computing dynamical processes. The latent heating may depend on the model resolution and the interval at which the precipitation physics is evaluated. Numerical results from a series of short range forecasts are compared to study the impact of varying the horizontal resolution and the interval for evaluating condensation physics, and of excluding the parameterized convective heating. A horizontal grid spacing of 40 km (coarse mesh) or 20 km (fine mesh) in National Centers for Environmental Prediction's Quasi-Lagrangian Model (QLM), and the initial data for a tropical storm case, are utilized. Resolvable scale condensation is invoked only at supersaturated grid points, and a Kuo-type convective parameterization procedure is employed. Significant structural differences are produced when the interval for computing both parameterized convection and resolvable scale heating is changed, and these differences broaden when the horizontal resolution is increased. The central warm cote structure and storm intensity are simulated better when both condensational processes are evaluated at an interval of twelve time steps than at each time step. Vertical columns in central storm area rapidly become convectively stable, and the maximum in vertical motion and strongest horizontal winds shift in the outer storm area, when both condensational processes are invoked at each time step. The central storm area remains conditionally unstable, and strongest winds develop close to the center, when both condensational processes are evaluated at intervals of twelve time steps. The central storm area remains conditionally unstable also in the fine mesh experiment in which the parameterized convective heating is excluded and the resolvable scale heating is evaluated at each time step. Intense vertical motion and vigorous heating develop in deep vertical columns, indicating that the heating on the convective scale is simulated as the resolvable scale heating. The vertical distribution of heating and the storm structure, during the first six hours in this case, are similar to those in the fine mesh run in which both condensational processes are evaluated at intervals of twelve time steps. However, the storm intensifies more rapidly after 6 h in the former than in the later case. Numerical results from additional experiments are presented to show that predicted storm structure is modified with a change in interval for invoking either or both condensational processes, and these circulation differences are not due to the initial spin up. Transfer of moisture and heat from low levels into the higher troposphere in cumulonimbus clouds takes place in several minutes. Above cited and other predictions from the QLM suggest that storm structure. intensity and motion in a mesoscale model are likely to, be improved when parameterized convective heating is included; however, a parameterization scheme that concurrently produces alterations in the entire model cloud depth should be invoked at intervals of several minutes.

A semi-implicit multiple-nested grid model for simulation of flow in a tropical storm

ABSTRACT .A three-layer three-dimensional, triply-nested primitive equation model. suitable to simulate tropical storm, has been designed. A grid telescopic technique has been used with a fine grid mesh of 18 km grid length in the centre which is surrounded by a medium mesh of 54 km grid length; this is again surrounded by a course grid mesh of 162 km grid length. Each mesh consists of 32 X 32 array of momentum points enclosing 31 X 31 array of mass points. The variables are staggered in space which reduces the amount of averaging to a minimum and hence improves accuracy. To suppress non-linear instability an improved finite difference scheme has been applied. A two-way interaction method has been adopt to match the solutions between grids of different lengths. To increase the time step for integration, a semi-implicit scheme has been used. The speed of the solution of the system of Helmholtz equations arising out of semi-implicit scheme has been appreciably increased by devising an iterative method. To examine the role of surface friction as postulated by Yamasaki (1977) and forced subsidence as hypothesized by Arnold (1977), Gray (1977) and Yanai (1961) at the initial stage of development of a tropical storm. numerical experiments have been accomplished with this model varying coefficient of surface drag. and specifying heat around the centre of the to disturbance which is considered as the effect of forced subsidence through an analytical function similar to one used by Harrison (1973). The integration was started from a weak barotropic vortex in &r8dient balance en and continued for 48 hours in two cases and 60 hours in one case. It is observed that surface friction may not be an essential factor at the initial stage of development of tropical storm when the vortex is weak. On the other hand, initial development could be initiated by forced subsidence. But in the subsequent stage, surface friction plays an important role to induce mass convergence in the boundary layer and to reduce horizontal of the disturbance. This preliminary experiment has yielded smooth and encouraging results.

The comparison of two parametric wind models for hurricane storm surge prediction

The tropical storm surge models depend critically on the maximum surface wind and shape of the wind profile. Since none of them are easy to measure, designing the parametric wind models for the storm surge prediction becomes divergent. Two widely used, but very different, wind models are examined. The study of their parameters showed that their resulting maximum wind and the shape of the wind profiles are similar. This property is a very useful guide for evaluating different surge models.

Highwater Mark Collection after Post Tropical Storm Dorian and Implications for Prince Edward Island, Canada

Prince Edward Island (PEI), Canada has been experiencing the consequences of a rising sea level and intense storms on its coasts in recent years. The most recent severe event, Post Tropical Storm Dorian (Dorian), began impacting Prince Edward Island on 7 September 2019 and lasted for over 20 h until the morning of 8 September 2019. The measurement of highwater marks (HWM) from the storm was conducted between 25 September and 25 October 2019 using a high precision, survey grade methodology. The HWM measured included vegetation lines, wrack lines, beach, cliff, and dune morphological features, and tide gauge data at 53 locations in the Province along coastal areas that are exposed to high tides, storm surge, high winds, and wave runup. Photos were taken to provide evidence on the nature of the HWM data locations. The data reveal that Dorian caused extensive coastal floods in many areas along the North and South Coast of Prince, Queens and Western Kings Counties of Prince Edward Island. The floods reached elevations in excess of 3.4 m at some locations, posing threats to local infrastructure and causing damage to natural features such as sand dunes in these areas. The HWM data can provide useful information for community and emergency response organizations as plans are developed to cope with the rising sea level and increased frequency of highwater events as predicted by researchers. As Dorian has caused significant damage in several coastal areas in PEI, better planning using an enhanced storm forecasting and coastal flood warning system, in conjunction with flood stage values, could possibly have reduced the impacts of the storm in the impacted areas. This could help enhance public understanding of the potential impacts in local areas and how they can prepare and adapt for these events in the future.

Wind Speed Analysis of Hurricane Sandy

The database of the HWind project sponsored by the National Oceanic and Atmospheric Administration (NOAA) for hurricanes between 1994 and 2013 is analysed. This is the first objective of the current research. Among these hurricanes, Hurricane Sandy was selected for a detailed study due to the number of files available and its social relevance, with this being the second objective of this study. Robust wind speed statistics showed a sharp increase in wind speed, around 6 m s−1 at the initial stage as Category 1, and a linear progression of its interquartile range, which increased at a rate of 0.54 m s−1 per day. Wind speed distributions were initially right-skewed. However, they evolved to nearly symmetrical or even left-skewed distributions. Robust kurtosis was similar to that of the Gaussian distribution. Due to the noticeable fraction of wind speed intermediate values, the Laplace distribution was used, its scale parameter increasing slightly during the hurricane’s lifecycle. The key features of the current study were the surface and recirculation factor calculation. The surface area with a category equal to, or higher than, a tropical storm was calculated and assumed to be circular. Its radius increased linearly up to 600 km. Finally, parcel trajectories were spirals in the lower atmosphere but loops in the mid-troposphere due to wind translation and rotation. The recirculation factor varied, reaching values close to 0.9 and revealing atmospheric stratification.